Percolation test and drainage field size (Vp calculator)
Work out your Vp, check it is inside the legal band, and size the drainage field with the official formula.
This is the test that decides everything else. Before anyone quotes you for a tank, the ground has to prove it can take the treated water — and the number it gives you, the Vp, is what the drainage field size is built from.
Approved Document H §1.44 gives the formula, verbatim: “To calculate the floor area of the drainage field (At in m²), the following formula should be used: At = p x Vp x 0.25” — p is people, Vp is your percolation value in seconds per millimetre. Read it on gov.uk (paragraphs 1.34–1.44).
Drainage field area for every Vp in the legal band
Worked with the Approved Document H formula, At = p × Vp × 0.25, so you can read your answer without the calculator. One pattern to notice: for a four-person household the area in square metres equals the Vp — a Vp of 40 means 40 m².
| Vp (s/mm) | 4 people | 6 people | 8 people |
|---|---|---|---|
| 12 | 12 m² | 18 m² | 24 m² |
| 15 | 15 m² | 22.5 m² | 30 m² |
| 20 | 20 m² | 30 m² | 40 m² |
| 30 | 30 m² | 45 m² | 60 m² |
| 40 | 40 m² | 60 m² | 80 m² |
| 50 | 50 m² | 75 m² | 100 m² |
| 60 | 60 m² | 90 m² | 120 m² |
| 80 | 80 m² | 120 m² | 160 m² |
| 100 | 100 m² | 150 m² | 200 m² |
| Under 12 or over 100 | No drainage field | No drainage field | No drainage field |
Band and formula: Approved Document H §1.38 and §1.44. The area is the floor area of the trenches, with 2m kept between parallel trenches.
The band matters more than the number. §1.38: drainage field disposal “should only be used when percolation tests indicate average values of Vp of between 12 and 100”. Below 12 the ground drains too fast and, in the document's own words, “untreated effluent cannot percolate too rapidly into groundwater” is precisely what the limit is there to prevent. Above 100 it barely drains at all. Outside that band, a drainage field is the wrong answer — not a smaller or larger one.
How the test is run is part of the rule, not a detail. §1.34: “A hole 300mm square should be excavated to a depth 300mm below the proposed invert level of the effluent distribution pipe.” §1.35: fill it and “allow it to seep away overnight” — skip that and you are timing dry ground, which lies. §1.36 is the actual measurement: refill, then “observe the time, in seconds, for the water to seep away from 75% full to 25% full level (i.e. a depth of 150mm). Divide this time by 150.” §1.37: “at least three times with at least two trial holes”, averaged, and never “during abnormal weather conditions such as heavy rain, severe frost or drought”. A test run in a dry August flatters the ground; the winter tells the truth.
Groundwater kills more drainage fields than soil type does. The document is blunt: “The groundwater table should not rise to within 1m of the invert level of the proposed effluent distribution pipes”, and the trial hole goes “a minimum of 1.5m below the invert”. Parallel trenches keep “2m wide being maintained between” them, and §1.43 puts an inspection chamber between tank and field.
FAQ
What is a good Vp value?
Anything between 12 and 100 seconds per millimetre. Approved Document H §1.38 allows a drainage field only inside that band: under 12 the effluent reaches groundwater too fast, over 100 the ground will not take it.
How big will my drainage field be?
At = p × Vp × 0.25 (§1.44), in square metres. Four people on a Vp of 40 need 4 × 40 × 0.25 = 40 m². The same house on a Vp of 80 needs 80 m² — the ground, not the household, sets the size.
Can I do the percolation test myself?
The method is public. A 300mm square hole 300mm below the pipe invert (§1.34); fill it and let it seep away overnight (§1.35); next day refill and time the fall from 75% to 25% full — 150mm — then divide the seconds by 150 to get Vp (§1.36); repeat “at least three times with at least two trial holes” and average (§1.37); never in heavy rain, frost or drought. Whether your Building Control accepts a self-run test is a separate question — ask before you dig.
Is a Vp of 13 good?
It passes, just. Approved Document H §1.38 allows a drainage field between Vp 12 and 100, so 13 is inside the band but close to the fast-draining limit. A four-person field would be 4 × 13 × 0.25 = 13 m². Make sure 13 is the average of at least three tests in at least two holes (§1.37): a single reading that close to 12 can fall on the wrong side of the line next time.
What does a Vp of 93 mean?
Slow ground that still qualifies. 93 is inside the 12–100 band of §1.38, close to the upper limit, beyond which the document says effective treatment is unlikely. The field gets large: 4 × 93 × 0.25 = 93 m² for four people, 139.5 m² for six. If the test was run in a dry spell, a winter re-test is worth having before you commit to that much trench.
Can I test with a hole bigger than 300mm square?
Not if you want the Approved Document H figure. §1.34 defines the hole — 300mm square, 300mm below the pipe invert — and §1.36 times a 150mm fall in that hole and divides by 150. A result from a different hole is a different test, so ask Building Control before you rely on it.
My Vp is over 100. Now what?
A drainage field is out. §1.38 says that outside the band “effective treatment is unlikely to take place in a drainage field”, and points to an alternative form of secondary treatment instead — which in practice means a sewage treatment plant, and a different conversation with the regulator.
Researcher & editor, off-mains drainage
Writes independent guides on septic tanks, cesspits and sewage treatment plants for homes off the mains. Cross-checks the general binding rules and the Environment Agency, SEPA, NRW and NIEA against real prices, British Standards and what owners actually report on the forums.